Symptoms
You will likely not notice any drivability problems, although there may be symptoms. The code may be more likely to occur after the engine has had numerous cold starts in the past 1-2 days.

Causes
A code P0421 may mean that one or more of the following has happened:

The catalytic converter is no longer functioning properly
An oxygen sensor is not reading (functioning) properly
A sparkplug is fouled up

Possible Solutions
Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

<--- Please fill in the 'location' and 'what you drive(year & chassis) and engine size' boxes on your personal profile page. This information makes it much easier for other members to answer/comment on your posts.

Add local shop labor, and possible bolts/studs/nuts as required, and gasket ot bottom of catalyst to front pipe..

Thank you (Spawn and sbr771) is there any pictures to show me how to change it and if there is any other solution because it seems expensive i have change my spark plugs from a month ago, dose spark plugs wires can cause that as well.

<--- Please fill in the 'location' and 'what you drive(year & chassis) and engine size' boxes on your personal profile page. This information makes it much easier for other members to answer/comment on your posts.

But i have checked with the code reader about mis-fire it was ok no mis-fire over heat i haven't experience that before with my vehicle

Meltdown i dont know what you mean with that !!

thank you

You stated in previous post you replaced plugs a month ago... was the engine running bad/bucking/jerking/MIL flashing ??

Misfire will kill a cat.. what happens is the raw fuel from cylinder gets dumped into cat, and burn there,, this in turn overheats the cat element, and it melts down... it is a ceramic honeycomb in a sense with precious metal to aid the reaction of the cat.. when the honeycomb overheats and melts down, it can clog the exhaust.

<--- Please fill in the 'location' and 'what you drive(year & chassis) and engine size' boxes on your personal profile page. This information makes it much easier for other members to answer/comment on your posts.

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